1. Field Of The Invention
The present invention relates generally to developing apparatus, and more particularly is directed to an apparatus for developing an image or picture on a sheet of developing paper from a photo-sensitive sheet of the type having a coating of photo-sensitive microcapsules.
2. Description Of The Prior Art
It is known, for example, as disclosed in Japanese Laid-Open Patent Gazette No. 59-30537, to print or develop an image of a color picture on a sheet of printing or developing paper from a photo-sensitive sheet having a coating of photo-sensitive microcapsules which has been irradiated with a light image of the picture. This known printing method employs, as the photo-sensitive sheet, a base made of a suitable resin on which there is coating of uniformly distributed yellow, magenta and cyan microcapsules. Each of the yellow, magenta and cyan microcapsules has an outer shell of a relatively soft material in which there is sealed an optically-hardening composition. The optically-hardening composition of each microcapsule contains a respective pigment or a precursor thereof. When the photo-sensitive microcapsules are irradiated with respective wave lengths of light the optically-hardening compositions therein are hardened. For example, the irradiation with blue light hardens the yellow microcapsules containing the precursor of yellow pigment, the irradiation with green light hardens the magenta microcapsules containing the precursor of magenta pigment, and the irradiation with red light hardens the cyan microcapsules containing the precursor of cyan pigment.
It will be appreciated that, when irradiating the above described photo-sensitive sheet with a light image of a color picture, the light of the color picture is suitable analyzed into the three primary colors of red, green and blue and the photo-sensitive sheet is irradiated separately with a red light image, a green light image and a blue light image by which the cyan, magenta and yellow microcapsules, respectively, are hardened in accordance with those images. After such irradiation of the photo-sensitive sheet, the latter is passed, in contact with a sheet of developing paper, through the nip between rotating pressure rollers which rotate about axes of right angles to the direction of sheet travel, and which extend across the full width of the sheet. The resulting pressure ruptures the shells of the yellow, magenta and cyan microcapsules that were not substantially hardened by irradiation with light of corresponding wave lengths. The substances containing precursors of pigments are transferred from the ruptured microcapsules to the facing surface of the sheet of developing paper which contains substances for developing the respective color pigment from the precursor thereof. Thus, the transferred precursors of pigments from the yellow, magenta and cyan microcapsules are suitably mixed to present a developed or printed color picture on the sheet of developing paper.
When the photo-sensitive sheet and the sheet of developing paper are pressed together between rollers rotating about axes at right angles to the direction of sheet travel therebetween, very large forces have to be exerted for urging together the rollers. More specifically, in order to rupture the unhardened microcapuules of the photo-sensitive sheet and to ensure adequate contact of the released pigment precursors with the developing substances on the sheet of developing paper, it is necessary that the photo-sensitive sheet and the sheet of developing paper be urged against each other with a pressure of 30 to 40 kg/mm.sup.2 In the case where, for example, the picture is to be printed on a sheet of developing paper of A4 size which has a width of 21 cm, and the rollers employed for pressing together the photo-sensitive sheet and the sheet of developing paper each have a diameter of 50 mm, the force urging the rollers together has to be about 57 kg for each cm of roller length, that is, a total force of 1200 kg for rollers having lengths of 21 cm. However, a developing apparatus capable of applying such a large force for urging together the rollers must be large and relatively complex in construction.
In order to solve the foregoing problem, it has been previously proposed, for example, as disclosed in Japanese Patent Application No. 62-93879, having a common assignee herewith, to provide a developing apparatus employing a dot impact head for applying pressure to microcapsules so as selectively rupture the latter in accordance with the selective optical-hardening thereof and to transfer the respective precursors of pigments to a sheet of developing paper. In such developing apparatus, the dot impact head is moved in a predetermined path or pattern and, at any time, applies impacts to only very small incremental areas of the photo-sensitive sheet so that the pressure required for rupturing the unhardened microcapsules can be achieved with the exertion of a relatively small force by the dot impact head. However, this developing apparatus has the disadvantage of being objectionably noisy when in use, and the useful life of the dot impact head is short.